This invention relates to a heat transfer surface for generating vapor from liquid, and particularly to a heat transfer surface having a single layer cover means disposed over a heating surface for diverting the vapor generated to flow along the heating surface and having openings for releasing freely the vapor out of the cover means.
Various heat transfer surfaces for enhancing nucleate boiling have been suggested in the art. There is a general concept in nucleate boiling that the heat transfer performance of a heat transfer surface can be enhanced by increasing nucleation sites on the heating surfaces or by inducing agitation near the heat transfer surface. Most of the heat transfer surfaces in the art relate to the type which produce nucleation sites such as small voids or cavities under a heating surface. Most vapor bubbles are generated in these voids or cavities. Restricted openings are provided for releasing the vapor generated in the cavities and for the entering of the liquid into the cavities. Such kinds of nucleation sites are vapor traps which trap the vapor generated. Only when the vapor pressure in the cavities increases to be greater than that of the liquid outside the cavities, do the vapor bubbles depart from the cavities. Examples of the references which disclose such heat transfer surfaces are U.S. Pat. Nos. 4,561,497, 4,606,405, 4,619,316 and 4,602,681 and Japanese Pat. No. 59-46490.
The heat transfer surface disclosed in U.S. Pat. No. 4,561,497 includes a plurality of rows of parallel void strip members between a base member and an outer surface region and restricted openings at both ends of each void strip member. The void strip members are formed by one or more layers of grooved thin plates laminated with the heating surface. The grooved thin plates must be bonded intimately to the heating surface, preferably with a metallurgical bond, to form the void strip members. The void strip members are intercommunicated through communication portions adjacent to the restricted openings so that vapor bubbles and vapor films are easily formed in the void strip members. The openings in the apparatus of this patent are formed by spacing the thin plate members and are restricted so that vapor can be released from the void strip members when the vapor pressure increases.
The heat transfer surface disclosed in U.S. Pat. No. 4,606,405 is also provided with narrow voids and restricted openings. The cross-sectional area of each void is greater than that of the restricted opening. The heat transfer surface disclosed in U.S. Pat. No. 4,619,316 is characterized in that, a heat conductive member which is installed on a heat generating body and has a plurality of layers of conductive plates which have narrow long cavity groups and apertures are stacked in a direction perpendicular to the surface of the heat generating body. The conductive plates include alternately stacked vertical cavity groups and horizontal cavity groups. Besides the above described differences of the arrangement from the present invention, the size of the apertures is small relative to the dimension of the cavity, and hence the vapour bubbles are restricted from flowing freely out of the cavities which perform the function of nucleation sites.
The heat transfer surface described in U.S. Pat. No. 4,602,681 also has a plurality of cells and restricted holes, and the cells are formed in a plurality of laminated layers in a direction from an outer surface of the heat transfer wall to an inside thereof.
Japanese Pat. No. 59-46490 discloses a heat transfer wall having primary cavity parts and primary openings in a lower layer and a secondary cavity parts in an upper layer. The upper layer which is formed by depressing the tops of the fins is employed for confining small cavities which are vapor bubble producing points or nucleation sites.
Generally, the fabrication of the heat transfer surfaces described above is complicated since particular configurations of narrow long cavities and restricted openings are necessary so as to achieve a desired effect. Furthermore, the parts for confining cavities must be formed as one piece with the heating surface or bonded intimately to the heating surface, thereby causing complication in the processing of the heat transfer surface. The quantities of the cavities are numerous and the density of the restricted openings on the heating surface should be very high. In addition, these cavities are liable to be clogged with impurities.